(a) Field of the Invention
The present invention pertains to an FM detecting circuit, for example for use in superheterodyne FM radio receivers.
(b) Description of the Prior Art
An FM detecting circuit, for example for use in superheterodyne FM radio receivers, in general, is a circuit for deriving an audio signal from an intermediate frequency output signal having been amplified by an intermediate frequency amplifying circuit. A known such FM detecting circuit is designed to form a signal having a phase shift in proportion to a frequency deviation from a carrier frequency, and the resulting signal is converted to a voltage to obtain an FM detection output signal. Such a type of FM detecting circuit includes, for one thing, the so-called ratio detection system which uses two electromagnetically coupled tuning circuits for producing therebetween a .pi./2 phase difference to utilize a variation of this phase difference caused by a modulated frequency, and for another thing, the so-called quadrature detection system which is arranged to effect switching, i.e. selection, of an FM input signal having a .pi./2 phase-shift from said FM input signal to obtain an output, utilizing the phenomenon that said phase-shift varies in proportion to the frequency deviation.
FIG. 1 shows a basic arrangement of a known ratio detecting circuit. In FIG. 1, reference numeral 1 represents a detecting coil unit which is comprised of a primary coil 1a, a secondary coil 1b which is electromagnetically coupled to said primary coil 1a for inducing a voltage with a .pi./2 phase-shift relative to the phase of a voltage applied to said primary coil 1a, and a tertiary coil 1c having one terminal connected to the mid point of the secondary coil 1b for inducing a voltage with a .pi. phase-shift relative to the phase of a voltage applied to the primary coil 1a. A capacitor 2 is inserted between the opposite terminals of the primary coil 1a. A resonance circuit which is formed by the primary coil 1a and capacitor 2 is adapted to resonate to a carrier frequency which may, for example, be 10.7 MHz or an intermediate frequency provided by a stage in an FM radio receiver assigned for supplying an output across two input terminals 3a and 3b. Also, a capacitor 4 is connected between the opposite terminals of the secondary coil 1b. A resonance circuit which is formed by the secondary coil 1b and capacitor 4 also is adapted to resonate to said carrier frequency. On the other hand, one of the terminals of the secondary coil 1b is connected to an anode of a diode 5, and the other terminal is connected to a cathode of another diode 6. Between the cathode of the diode 5 and the anode of the diode 6 are connected capacitors 7 and 8 which are series-connected to each other and which have a same capacitance value, series-connected resistors 9 and 10 having a same resistance value, and a capacitor 11 having a large capacity. Furthermore, a connecting point C of the capacitors 7 and 8 is connected to the other terminal of the tertiary coil 1c, and also to an output terminal 12a. A connecting point of the resistors 9 and 10 is connected to another output terminal 12b and to ground.
The known ratio detecting circuit mentioned above is operative so that, when an FM input signal is applied across the input terminals 3a and 3b, there are induced a voltage V.sub.A and a voltage V.sub.B whose ratio is adapted to vary in accordance with a frequency deviation of an FM signal, across a terminal point A of the secondary coil 1b and the other terminal point C of the tertiary coil 1c, and across the other terminal point B of the secondary coil 1b and said point C, respectively, of the detection coil unit 1. These voltages V.sub.A and V.sub.B are subjected to detection by the diodes 5 and 6 so that a voltage V.sub.D is produced across the point D located on the cathode side of the diode 5 and said point C, and another voltage V.sub.E is produced across the point E located on the anode side of the diode 6 and said point C, respectively. A differential voltage between these voltages V.sub.D and V.sub.E is derived, as a detection output, across the output terminals 12a and 12b. In the above-stated detection circuit of the prior art, it should be understood that, in case the signal level of the FM input signal input across the input terminal 3a and 3b is high, there arises the tendency that the voltage, V.sub.D -V.sub.E, between the points D and E becomes higher. At such instance, however, there flows a large charge-up current for the capacitor 11, so that, as a result, said voltage V.sub.D -V.sub.E is held constant. Also, even in case the signal level of the FM input signal becomes low and in case, accordingly, the voltage V.sub.D -V.sub.E tends to become low, this voltage V.sub.D -V.sub.E is held constant owing to the electric charge of the capacitor 11. Thus, this known ratio detecting circuit has in itself a voltage or signal level limiting function by the provision of the capacitor 11.
It should be understood, however, that the instance wherein said signal level limiting function operates in the ratio detecting circuit of the prior art stated above is limited to such instance where the velocity of variation of the signal level is greater than a time constant which is determined by the value of the capacitor 11 and the values of the resistors 9 and 10. In case the velocity of variation of the signal level is smaller than the time constant, there is not developed any signal level limiting function in this detecting circuit.
Also, the ratio detecting circuit of the prior art described above has another disadvantage in addition to that discussed above. When consideration is made of an output voltage characteristic, i.e. S-curve characteristic, relative to an input frequency, the rectilinear region in the S-curve characteristic is only within the range of .+-.150 kHz, and thus no sufficient distortion-reducing effect can be expected.
On the other hand, in the known quadrature detecting circuit, its .pi./2 phase-shifting circuit is, ordinarily, comprised of a coil and an LC resonance circuit whose resonance frequency is identical with the frequency of a carrier signal. This detection circuit utilizes a resonance characteristic in the vicinity of a resonance frequency of said LC resonance circuit in order to effect a .pi./2 phase-shifting of an FM input signal. Thus, this known quadrature detecting circuit has the disadvantage that, in case the resonance characteristic of the LC resonance circuit is not proper, i.e. in case the rectilinear region of the resonance characteristic in the vicinity of the resonance frequency is narrow, the distortion characteristic in the detected signal becomes poor. Accordingly, it is usual in this known detecting circuit to provide means of some sort as a counter-measure for the purpose of suppressing the development of the distortion at the time of detection due to said resonance characteristic.
FIG. 2 is a circuit diagram showing an example of a known quadrature detecting circuit designed so as to suppress the development of the aforesaid distortions. In FIG. 2, reference numeral 13 represents a phase-shifting circuit for effecting a .pi./2 phase-shift of an FM input signal applied across input terminals 14a and 14b. This phase-shifting circuit 13 is comprised of a coil 15 having an end connected to the input terminal 14a and of an LC circuit 18 inserted between the other end of said coil 15 and the input terminal 14b and formed with a coil 16 and a capacitor 17, and a resistor 19 which is connected in parallel with said LC resonance circuit 18. By arranging the resonance frequency f.sub.o of this LC resonance circuit 18 to be in agreement with the carrier frequency, there is derived a signal which represents a .pi./2 phase-shifted FM input signal, across the opposite terminal points F and G of the LC resonance circuit 18. Reference numeral 20 represents a multiplying circuit for effecting switching of said FM input signal by an output signal of the phase-shifting circuit 13. This multiplying circuit 20 is a double-balanced differential circuit provided with three differential amplifiers 27 to 29 which are formed by transistors 21 to 26. A product output signal of this multiplying circuit 20 is output to its output terminals 30a and 30b.
In the above-stated quadrature detecting circuit, an FM input signal which is input across the input terminals 14a and 14b is directly supplied to the multiplying circuit 20. Simultaneously, this FM input signal is supplied to this multiplying circuit 20 as a signal having been given a .pi./2 phase-shift by the phase-shifting circuit 13. The multiplying circuit 20 effects switching of the FM input signal with a signal representing a .pi./2 phase-shifted FM input frequency signal, and outputs, across the output terminals 30a and 30b, a pulse signal having a pulse width which is in proportion to a phase difference between the signal applied across the input terminals 14a and 14b and the signal across the point F and G. The signal which is derived across the output terminals 30a and 30b is rendered to an FM detection output by passing this signal across the terminals 30a and 30b through a low-pass filter not shown.
The above-stated quadrature detecting circuit is arranged so that the development of distortions at the time of detection due to the resonance characteristic of the LC resonance circuit 18 in the phase-shifting circuit 13 is suppressed by damping the quality factor Q of this LC resonance circuit 18. More specifically, the frequency characteristic versus phase in the phase-shifting circuit 13 can be varied in a manner as shown in FIG. 3 by varying the quality factor Q of the LC resonance circuit 18. By damping the quality factor Q, it is possible to expand the rectilinear region of the resonance characteristic of the LC resonance circuit 18. As described above, this known quadrature detecting circuit is designed so that the quality factor Q of the LC resonance circuit 18 is damped to expand the rectilinear region of the frequency characteristic of the phase-shifting circuit 13, to thereby suppress the development of distortions at the time of detection.
The above-described known quadrature detecting circuit, however, has the disadvantage that the detection efficiency, i.e. the ratio of output voltage deviation to input frequency deviation, becomes poor because of the arrangement that the quality factor Q of the LC resonance circuit 18 is damped by means of the resistor 19. There is a further disadvantage that, in view of the effect of expansion of the aforesaid rectilinear region of the resonance characteristic by said resistor 19 being trivial, then no sufficient distortion-reducing effect can be expected.
FIG. 4 is a circuit diagram showing another known quadrature detecting circuit designed to reduce the development of distortions at the time of detection. The quadrature detecting circuit shown therein is such that the phase-shifting circuit 13 is arranged in a double-tuning fashion. A tuning circuit 31 is constructed so that a capacitor 33a and a resistor 34a are connected in parallel to the primary coil 32a of a coil unit 32, and a capacitor 33b and a resistor 34b are connected in parallel to the secondary coil 32b of said coil unit 32. According to this known quadrature detecting circuit, it is possible to expand the rectilinear region of the frequency characteristic in the phase-shifting circuit 13 without the accompaniment by a lowering of detection efficiency, by appropriately selecting the coupling coefficient of the primary coil 32a and the secondary coil 32b, the resistance values of the resistors 34a and 34b, and the respective resonance frequencies of the respective resonance circuits formed with the primary coil 32a and the capacitor 33a, and with the secondary coil 32b and the capacitor 33b, respectively.
The above-stated quadrature detecting circuit has the disadvantage that the aforesaid selection of respective values of said constituent elements is very troublesome, and a further disadvantage that, alike the prior circuit shown in FIG. 2, the effect of expansion of the rectilinear region of the frequency characteristic of the phase is only small, so that no sufficient distortion-reducing effect can be expected.
As explained above, both of the above-described known ratio detecting circuit and quadrature detecting circuit have the disadvantages that the rectilinear region of the resonance characteristic of the resonance circuit in the vicinity of the resonance frequency is narrow, and that, accordingly, the effect of reduction of distortions is not sufficient, so that there cannot be obtained a sufficient detection characteristic for a high frequency spectrum component which is required in performing the detection of FM multiple broadcasting. Also, in the known ratio detecting circuit, the signal level limiting effect is not sufficient.